FIG. 9 is a diagram illustrating a schematic structure of a conventional laser display.
The laser display 900 includes laser sources 901a˜901c corresponding to three colors of R, G, B, and light modulators 905a˜905c for intensity-modulating laser lights La˜Lc emitted from the laser sources 901a˜901c according to primary color signals Sa˜Sc of an input video signal. Further, the laser display 900 includes a mirror 903 for reflecting the laser light La modulated by the optical modulator 906a, a dichroic mirror 902a for multiplexing the laser light La reflected at the mirror 903 and the laser light Lb modulated by the light modulator 906b, a dichroic mirror 902b for multiplexing the laser light La from the dichroic mirror 902a and the laser light Lc modulated by the light modulator 906c, and a condenser lens 904 for condensing the laser light multiplexed by the dichroic mirror 902b. Further, the laser display 900 includes a polygon scanner 906 for scanning the laser light condensed by the condenser lens 904, on a screen 908 in x direction, and a galvanometer scanner 907 for scanning the light from the polygon scanner 906 on the screen 908 in y direction so as to form a two-dimensional image. A two-dimensional beam scanning means of this laser display 900 is constituted by the polygon scanner 906 and the galvanometer scanner 907.
Next, the operation will be described.
The laser lights La˜Lc from the laser sources 901a˜901c corresponding to the three colors of R, G, and B are intensity modulated by the light modulators 905a˜905c in accordance with the respective primary color signals Sa˜Sc of the input video signal, and multiplexed by the optical system comprising the mirror 903 and the dichroic mirrors 902a and 902b, and the multiplexed light is applied to the condenser lens 904. Further, the laser light condensed by the condenser lens 904 is scanned in the x direction of the screen 908 by the polygon scanner 906 and is scanned in the y direction of the screen 908 by the galvanometer scanner 907, whereby a two-dimensional image can be displayed on the screen 908.
In the laser display 900, since the lights from the R, G, B light sources are high output monochromatic lights, respectively, a bright image of a high chromatic purity can be displayed by using laser sources of preferable wavelengths.
Another characteristic of the two-dimensional scanning type laser display shown in FIG. 9 is that uniform illumination can be obtained, without using a complicated optical element such as an integrator, in the optical system from the two-dimensional beam scanning means to the screen 908. For example, in a projector using a discharge tube or the like as a light source, which is presently available in the market, illumination intensity is made uniform using a light integrator comprising two lens arrays. However, the two-dimensional scanning type laser display can realize uniform illumination, regardless of the intensity distribution of the beam emitted from the light source, without using a large optical element for making the illumination intensity uniform.
Still another characteristic of the two-dimensional scanning type laser display is that, since the two-dimensional beam scanning means is used, a high-resolution and high-luminance image can be obtained. In a laser display using a spatial light modulator, a liquid crystal panel is generally used as a spatial light modulator. In this case, however, the beam power of the light source decreases due to scattering and absorption of light by the liquid crystal, and thereby the luminance of the light from the light source on the screen is deteriorated. On the other hand, in the laser display using the two-dimensional beam scanning means, a high-resolution image can be obtained by increasing the beam scanning rate. Further, since the laser display includes no optical system that decreases the laser power, the utilization efficiency of the laser power is high, and thereby a high-luminance image can be obtained.
In this laser display, however, there occurs a problem of so-called speckle noises in which bright-dot and dark-dot patterns are distributed at random, which are caused by that the high-coherence laser sources are used as light sources. The speckle noises are minute uneven noises which are caused by interferences of light beams which are scattered at the respective portions on the screen 908, when the laser light is scattered on the screen 908.
As a method for removing such speckle noises, there has conventionally been adopted a technique of changing the speckle patterns in a time shorter than the rewrite time of a display that can be sensed by human beings to average the speckle patterns, thereby preventing eyes of an observer from sensing the speckle noises.
For example, Patent Document 1 (Japanese Published Patent Application No.Sho.55-65940) discloses a method of eliminating speckle noises by vibrating a screen. Further, Patent Document 2 (Japanese Published Patent Application No.Hei.6-208089) discloses a method of eliminating speckle noises by disposing a diffusion element for diffusing coherent light on an optical path of the coherent light, and vibrating and rotating the diffusion element with an external force. Further, Patent Document 3 (Japanese Published Patent Application No.Hei.3-109591) discloses a method of preventing occurrence of speckle noises by applying particles of a birefringent crystal body to a screen, and projecting laser light whose polarization state is temporally varied, onto the screen.